I've moved our conversation to a separate topic to keep the iTunes Web Sources discussion on topic.
You're probably looking in
/Library/Application Support/...
and not at
~/Library/Application Support/...
which is the Library folder in your user folder, e.g.,
/Users/Aurelien/Library/Application Support/...
If you don't see the "Library" folder in your user's home directory, you can probably navigate to it via
Finder > ⇧⌘G
and then enter
~/Library/Application Support/
To install a Tag Source, simply download the archive from the respective topic on the forums, extract in (in case it's a *.zip file) and copy the *.src file to the folder mentioned above:
/Users/<Your User Name>/Library/Application Support/de.mp3tag.Mp3tag_<Some Number>/drive_c/users/Mp3tag/data/sources
Kind regards
– Florian